What do you all think of stainless steel heat sheilds?

I have one but i havent put it back on since i fitted the GRS,i feel its going to stop the heat from escaping through the vents properly and keep too much heat floating around the back of the radiator...? This being a bad thing,but then why is it called a heat sheild?Whats it sheilding the heat from?The spark plugs?

Cheers